Lab Report: Experiment on friction in Pipe
Fundamental understanding of fluid flow is important to every industry related to chemical engineering, manufacturing industries etc. The flow networks are required to achieve continuous transport of products and raw materials from different processing units. Energy is required to be applied to the fluid to make it flow through the pipe overcoming energy loss in pipe. The pipe losses in a piping system takes place due to a number of system characteristics such as pipe friction, changes of flow direction, obstructions in flow path, sudden or gradual changes in cross-section and shape of flow path. In this report loss due to friction has been discussed. This kind of loss is also called frictional head loss or frictional pressure drop. The frictional loss takes place due to the friction between the fluid and the pipe wall and internal friction within the fluid. Substantial energy is lost due to frictional resistances in pipe flow.
Determination of pressure losses is needed to evaluate appropriate size pump. Knowledge of the magnitude of frictional losses is required to determine the power requirements of the pump forcing the fluid through the pipe.
Aims and Objectives:
In this experiment friction in Pipe has been studied. So, the purposes of this experiment are to investigate the relationship between friction factor and Reynolds number for flow through smooth pipes as well as to ascertain whether the flow is laminar or turbulent and compare the friction factors with those from Moody diagrams.
Methodology and Theory:
Compressible Flow Rig is used in this experiment. This rig has a flow bench with various flow conduits made up of Perspex. Through a converging nozzle flow enters into the conduit. Pressure drop at the entrance is used to find out air velocity. The rig is attached to suction side and for this reason flow is free from swirl and undue fluctuation. Picture of the rig is shown below:
At first a pipe section was attached to the measuring nozzle and the intake connection using union nuts. It was to be noted that the longer end was the abatement section for the inlet and itwas connected to the measuring nozzle. Then 34 mm dia pipe was connected to the differential pressure manometer of measuring range 0-25 mbar as well as 24mm and 16 mm pipe to manometer of measuring range 0-200 mbar. After this, pressure measurement point of the measuring nozzle was connected to the negative connection of the velocity display. Then compressor was switched on and desired speed was set. The velocity Vo from the nozzle and the pressure loss Δp was recorded in data sheet. In the next step, a new velocity was set and whole measurements were repeated. Then the pipe was changed and experiment was repeated for two other pipes.
While fluid flows through a pipe, roughness of internal wall of pipe can cause local eddy currents within the fluid resulting resistance of the flow of the fluid. From The velocity profile in a pipe will it can be found out that fluid elements in the center of the pipe move at a higher speed than those closer to the wall. So, friction occurs between layers within the fluid. Pipes having smooth walls such as glass, polyethylene have only a small effect on the frictional resistance. On the other hand pipes with rough walls such as concrete, cast iron and steel results larger eddy currents having a significant effect on the frictional resistance. Rougher the inner wall of the pipe, more will be the pressure loss due to friction.
With the increase of average velocity pressure losses increases and velocity is directly proportional to flow rate.
Velocity=Volumetric flow rate /Cross sectional area of the pipe
In case of smaller pipe greater proportion of fluid is in contact with the pipe resulting friction. Velocity is also influenced by pipe size. For a constant flow rate, reducing pipe size increases the velocity causing friction. Highly viscous fluid flows more slowly and does not support eddy currents and therefore the internal roughness of the pipe does not have effect on frictional resistance. This incident occurs in case of laminar flow. There are three stages of flow in a pipe. In each of these stages flow behaves in different manners in terms of their frictional energy loss and there are different equations that predict their behavior.
Mass flow rate, m at the nozzle entry is expressed by
m = AρVo ,
where, A = Inlet nozzle area = 0.0009078 m2
ρ= density of air = 1.2 kg/m3
Vo= velocity of air at the nozzle.
Reynolds number is expressed by ,
where, Vpipe was calculated from the mass flow rate and the area of the pipe.
Viscosity of air, μ =1.8x10-5 kg/m.s
Friction factor is calculated by using the Darcy’s equation.
f = 2.Δp.d/(ρ. Vpipe2)
h is pressure drop in terms of ‘head loss in metres of the fluid’ , h = Δp/( ρ.g)
Laminar flow generally occurs in case of small pipes, low flow velocities and with highly viscous fluids. There are neither cross currents nor eddies. For laminar flow, Re < 2300, friction factor (f) =64/Re, which is independent of relative roughness. So, there is linear relationship between Re and friction factor.
When Reynolds numbers is large, friction factor is independent of the Reynolds number. This type of flow is turbulent flow. Transitional flow is a mixture of laminar and turbulent flow and this flow occurs when values of Re is greater than 2100, but less than 4000.
Result: Tables and Graphs
The graphs for pressure and mass flow have same characteristics for flows through all three pipes. Initially pressure is independent of mass flow but at later stage with increase of pressure mass flow has increased. For flow through 34 mm dia pipe, mass flow has drastically increased due to slight increase of pressure. But for flow through 16 mm dia pipe, increment of mass flow is very small with increase of pressure.
It is to be noted that even for smooth pipes the friction factor is not zero. So in any pipe there is a head loss, no matter how smooth the surface is made. There is always some microscopic surface roughness that causes some head loss.
Moody chart covers wide range in flow parameters. The non-laminar region i.e. turbulent stage covers maximum region corresponding to Reynolds number from Re = 4 x 10^3 to Re = 10^8. It can be obviously said that for a given fluid and pipe, typical values of the average velocity do not generally cover this wide range.
Determination of energy loss in pipe flow due to friction is very important and it has significant use in practical field of engineering. Sometimes this energy loss is expressed as a pressure drop, sometimes as a head loss. Civil engineers often work with head loss, whereas chemical and mechanical engineers often work with pressure drop.